The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Two types of the mono-sheet DTR offset printing plate exist. According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the useless emulsion layers.
According to a second type of mono-sheet DTR offset printing plate a support, commonly anodized aluminium, is provided in the order given with a layer of physical development nuclei and a silver halide emulsion layer. After information-wise exposure and development the imaged element is washed with water to remove the emulsion layers so that a support carrying a silver image is left which is used as a printing plate. Such type of lithographic printing plates are disclosed in e.g. EP-A-278,766 and EP-A-410,500.
When using such type of mono-sheet DTR offset printing plate it has been found that when the emulsion is swollen with developing liquid sticking of the emulsion layer to the transporting rollers often occurs. This may cause image failures. In particular a bad ink acceptance, scumming, and low printing endurance of the plate may be noted.